But if he had been watching, instead of swimming, he would have seen a beautiful sight.

As dawn broke, the local winter river fog known as the Bridgewater Jerry raced downstream.

To the beat of Buddhist drums and clash of cymbals, the swimmers lined up along a small beach at suburban Sandy Bay.

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Pale bodies girdled with white towels and wearing red bathing caps, they stood as a uniform group waiting for the beat to peak, suddenly stop, and red flares to fire. Then as a whooping mass they waded out.

Rather than being a perv at nudes, the sight gave pause to think about human togetherness.

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For a moment, the nude swimmers of Hobart had the impact that photographer Spencer Tunick gave to massed reclining bodies on Princes Bridge, Melbourne, in 2007; and on the Sydney Opera House steps in 2010.

For 84-year-old Margaret Clougher, of nearby Taroona, who swims regularly in the Derwent, the idea of a mass of naked people around her on this particular swim was irrelevant.

"I didn't even think about it," Ms Clougher said.

The swim that was nearly banned also left its 203 participants mainly exhilarated.

"It was amazing. It was freezing. It was totally in the moment," said public servant Fiona Kotsidis, of Eaglement in Melbourne. "The body took over. It was on autopilot. It felt like it was burning rather than freezing."

Architect Michael Lewarne, from Newtown in Sydney, swam out to do something that he said had to be done at least once in life.

"In Sydney, this would not be as significant as Hobart - because of the cold," he said.

Organisers said the water temperature was a balmy 12 degrees, compared to two degrees of air temperature on frosted ground.

Being out of the water was harder than swimming for Fitzroy North artist Lauren Bell.

Organisers set up a ring of lifesavers on boards in the water and first aid stations on shore, including a heated emergency room equipped with defibrillators. Swimmers were told it was safest to immerse slowly.

Police, who initially banned the swim but later relented under an agreement with the festival organisers and council, said it had unfolded without a hitch.

"This was done in a good context," Inspector Glen Woolley said. "But if someone turns up tomorrow and swims naked, they'll be charged."